Welcome to the gang, colesey. At junior levels, they won't be trying to con you (too much), so no need to alert them to you status - just referee what you see.
Despite your law knowledge, you WILL make mistakes, and be hugely embarrassed about them for years to come. That goes with the territory. Expect that your brain will go into overload as the game moves on while you are still trying to process what happend a few moments ago - that is also very normal. If you feel overwhelmed (also normal) just concentrate on making it safe and calling the easy spots - knock-on, forward pass etc. At junior levels, with any luck only you will know if you are winging it.
Do swot up on in-goal; it's an area that few players are knowledgeable on (so your knowledge from playing days may not help you), and where decisions can be counter-intuitive. Rememebr - focus on WHO TOOK IT IN - not so much on what happened next.
U.13 and U.14 are an unalloyed delight - enjoy the games, enjoy being involved, adn get your fitness and skills up before you take the plunge in March.
Best of luck